Norwegian mass murderer Anders Behring Breivik has changed his name to Fjotolf Hansen, his lawyer has confirmed.
The move came a day after the country's Supreme Court refused to hear the neo-Nazi's appeal over "inhumane" prison conditions.
Breivik murdered 77 people in 2011 in two terror attacks that shocked Norway.
Lawyer Oeystein Storrvik refused to share Breivik's reasons for choosing the name Hansen, one of Norway's most common surnames, or the rarer Fjotolf.
"He told me some reasons but I don't want to talk about what he told me," Mr Storrvik told Reuters.
Under Norwegian law, people can change their names in the official register, as long as they will not cause offence, or harm the person concerned.
It is not known when Breivik decided to change his name. However, the Norwegian business register shows that Breivik Geofarm, an agricultural firm the killer used to obtain bomb-making materials, is now registered to Fjotolf Hansen.
According to Norway's Statistics Bureau, there are more than 52,000 people with the surname Hansen in a population of five million.
It says that "fewer than four people, or no-one" use the first name Fjotolf.
Breivik, a far-right extremist, killed eight people in Oslo with a bomb, and then gunned down 69 others - many of them teenagers - at a summer youth camp on the island of Utoya.
Before carrying out the attacks he circulated a 1,500-page manifesto signed "Andrew Berwick" - the Anglicised version of his name.
Breivik was convicted of terrorism and premeditated murder, and given the maximum sentence of 21 years' imprisonment, which can be extended indefinitely.
He has never expressed any remorse for committing Norway's worst peace-time atrocity.